A central issue in behavioral neuroscience is how alterations in neural pathways mediate the durable behavior changes involved in learning. Taste aversion conditioning is an excellent model for studying the neural changes involved in learning because this conditioning can occur in a single trial, despite lengthy delays between conditioned and unconditioned stimuli. The proposed studies are based on the identification of a cellular correlate of the behavioral expression of a conditioned taste aversion, namely, cells in the nucleus of the solitary tract. C-fos induction occurs in cells in the nucleus of the solitary tract in response to a taste made aversive by conditioning, but not in response to the same taste prior to conditioning or to a taste (quinine) which is innately aversive. Proposed studies will combine this cellular measure with behavioral assessment to further assess the cellular c-fos response and its reliability as a marker of learning. Studies will also examine the functional importance of cells in the nucleus of the solitary tract which display c-fos induction during expression of this learning, using asymmetrical lesion techniques. Additionally, studies will continue to define the forebrain pathways critical to this learning, with a focus on ipsilateral connections between the amygdala and the nucleus of the solitary tract which appear necessary for cellular, as well as behavioral, manifestations of this learning. The role of insular cortex will also be examined. Finally, studies will identify the targets of activated modified behavioral response to a taste after conditioning. By defining the neural pathways and cell types involved in CTA learning, this project provide the groundwork for eventually characterizing the plastic changes within and between cells which underlie this learning.